Upper and lower plate juxtaposition, deformation and metamorphism during crustal convergence, Trans-Hudson Orogen (Quebec-Baffin segment), Canada

The lower plate of the Quebec-Baffin segment of Paleoproterozoic Trans-Huds on Orogen comprises parautochthonous Archean crystalline basement of the Su perior Province (3.22-2.74 Ga) and imbricated Paleoproterozoic riftmargin s trata (2.04-1.92 Ga). The collisional upper plate contains (1) allochthonou s ophiolitic units (2.00 Ga), (2) a continental are terrane (Narsajuaq are; 1.86-1.82 Ga), (3) shelf sequence strata (ca. < 1.93 Ga and >1.86 Ga) and basement orthogneisses (ca. 1.95 Ga), (4) foreland basin deposits (<1.93 Ga and > 1.86 Ga), and (5) a batholith which intrudes the shelf and foreland basin rocks (Cumberland batholith; 1.86-1.85 Ga). Lower and upper plate ass emblages were juxtaposed during a 1.82-1.79 Ga accretion-collision event, a nd deformed into regional-scale folds during two post-collisional folding e pisodes (ca. 1.76-1.74 Ga). As a result of the structural relief generated by the post-collisional folding and subsequent exhumation, all orogenic ele ments can be directly examined along a 400 km, across-strike transect. In the external zone (northern Quebec), lower plate rift-margin units were internally imbricated (Cape Smith Belt) and translated southward across the Superior Province basement. Paleoproterozoic amphibolite facies mineral gr owth is syn to post-thrusting. Tectonic imbrication has resulted in a struc tural inversion of the accreted (i.e. upper plate) ophiolite stratigraphy. North of the Cape Smith Belt, mid-crustal plutonic units of the Narsajuaq a re form an upper plate crystalline wedge whose sole thrust is localized nea r the thrust belt's basal decollement. Within Narsajuaq are, syn-magmatic g ranulite facies assemblages are retrogressed to amphibolite facies assembla ges in proximity to major accretion-collision faults. In the internal zone (southern Baffin Island), a tripartite orogen-scale tectonostratigraphy is exposed on the flanks of the post-collisional fold structures. At the lowes t structural level, lower plate Superior Province orthogneisses are imbrica ted with Paleoproterozoic rift-margin units, and mineral growth is syn-thru sting. At intermediate structural level, upper plate orthogneisses are corr elated with the Narsajuaq are in northern Quebec. At the highest structural level, a north-facing shelf sequence is structurally overlain by an assemb lage of elastic rocks interpreted as a foreland basin sequence. Dominantly- monzogranite plutons of the Cumberland batholith are restricted to the uppe r structural level and indicate pre-emplacement imbrication of tectonostrat igraphic units. Within both upper and lower plates, granulite facies assemb lages are retrogressed to upper amphibolite assemblages in proximity to 1.8 2-1.79 Ga accretion-collision structures. Exposures of Superior Province basement across the orogen document a progre ssive increase in the extent of reworking during accretion of upper plate u nits, from a narrow shear zone in the external zone to km-scale imbrication and more pervasive shear deformation in the internal zone. A minimum of 16 0 km of basement underthrusting is constrained by the structural overlap of upper plate allochthons on lower plate units. Basement involvement during post-collision shortening is manifest as thick-skinned folding, indicating coupling across the collisional basal decollement. (C) 1999 Elsevier Scienc e B.V. All rights reserved.